Mr. Wallace Science 16'-17' - Air Pressure and Wind

Lecture Power Point Chapter 19:
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Study Guide:
19.1

Air pressure
- definition, cause, atmospheric profile
- how do we measure/demonstrate air pressure?
Generation of Wind
- Because the Earth's surface varies from place, and the direct sunlight varies due to Earth's curvature, it is heated unequally, causing air to lift in certain places, which leads to low pressure zones.
- Air from high pressure zones rushes in to fill up the empty space left behind by the rising air. This movement is wind.
- Pressure gradients and wind speed
Weather Maps
- Isotherms
- Isobars
- Weather Station Models
- Pressure Centers
Coriolis effect
- changes direction of high altitude winds
Friction
- effects direction and speed of wind near the Earth's surface
Jet Streams: high altitude "rivers" of very fast moving air. In the northern hemisphere, jet streams typically flow from West to East. In the N. Hemisphere's winter, the polar jet stream (between the North Polar Cell and the North Ferrel Cell) dips to lower latitudes across the U.S., bring colder air and contributing to snowy weather.
- Jet Streams

19.2

Pressure centers:
- Low pressure centers = cyclones = cloudy conditions and precipitation
  - Air moves across the Earth's surface from high to low pressure zones, moving inward towards the center of the low and then rises up into the sky, leading to clouds and possibly precipitation
  - Low Pressure zones are like a vacuum, air gets sucked up into the sky from the ground!
  - Must be balanced by diverging winds at higher altitudes: since air rises from a low pressure, there has to be room for that rising air above, or else the low pressure zone will not persists and break down.
- High pressure centers = anticyclones = clear skies and fair weather (not necessarily warm!)
  - Air moves from high altitudes down to the surface, creating a high pressure zone. The air can only spiral outwards across the surface of Earth from that point on.
  - Think of high pressure zones as if I poured a bucket of water out onto the classroom floor. Since the floor does not absorb the water (air), the water flows outwards in all directions.
  - Must be sustained by converging winds at higher altitudes - air must come together and be forced downward to the surface to maintain a high pressure zone.
- Rotation of air around pressure centers
Pressure systems generally move from West to East across the U.S., because our country primarily sits inside a pressure belt which is dominated by the Prevailing Westerlies ("wester-lees").
General pattern of air movement on Earth:
- Air is heated most around the equatorial region on Earth, rises and migrates towards the poles in each hemisphere, where it cools and sinks. Completion of this simple large convective cell occurs as the sinking cold polar air travels near the surface back toward the equator, where it is reheated.
Actual pattern of air movement on Earth (generalized for simplicity):
- Air near the equator is heated by the most direct sunlight of any latitude on Earth, so it rises, creating a belt of low pressure as-well-as a belt of precipitation around the globe. Why are all of Earth's best jungles and rain forests near the equator? Now you know! Its warm, sunny, but also receives a lot of rain throughout the year due to the equatorial low pressure belt.
- Air sinks near the 20-30 degrees latitude region, creating an arid, dry high pressure zone. Around this latitude, most of Earth's deserts can be found due to this high pressure zone. Air flows outwards from this high pressure zone, both north and south, producing in the Norther Hemisphere the equatorial trade winds (which blow east to west due to the coriolis effect) and the prevailing westerlies (which blow west to east). *Remember, winds are named for which way the blow FROM.
- Cold polar air sinks and makes its way towards the equator. This cold air meets the relatively warm air from the subtropical high and creates, primarily due to frontal wedging, another low pressure zone called the subpolar low pressure zone. Air rises here, making for cloudy/stormy conditions.
- YOU SHOULD BE FAMILIAR WITH THESE GLOBAL AIR MOVEMENT PATTERNS. SPEND TIME WITH THE DIAGRAMS, IT WILL MAKES SENSE. ALSO CHECK OUT:
Continental Influence:
- Because land heats up and cools down more quickly that water, the continents develop more high pressure zones during the winter (Norther Hemisphere) and more low pressure zones during the summer. The low pressure zones suck up moisture with the air traveling off the oceans. During the winter, the high pressure zones that develop force the air off the land generally, creating dry and cold conditions. These seasonal changes with pressure zones and wind flow are called monsoons. There are summer and winter monsoons, and the effect is more drastic in the Southern Hemisphere.